The present invention relates to a method and a device for forming a deployment criterion for a restraint system in a motor vehicle, where the acceleration of the vehicle in one or more directions is measured, and a criterion is derived from the measured acceleration for deciding whether or not the restraint system should be deployed.
A method is described in European Patent 458 796 in which, vehicle acceleration is first measured and then converted into a speed by integration over time. The deployment criterion is a threshold value which can be varied as a function of one or more state variables of the vehicle derived from the crash process. The restraint system is deployed when the measured vehicle speed exceeds this threshold. International Patent Application WO 94/14638 describes a method of forming a deployment criterion for a restrain system, where at least two deployment threshold values for the vehicle deceleration are given fuzzy definitions in a fuzzy method.
U.S. Pat. No. 5,673,365 describes a method where fuzzification of the vehicle acceleration is used to decide whether or not the vehicle has been involved in a crash.
The main requirement of a restraint system deployment algorithm is reliable deployment. This means that the restraint system is deployed in all crashes where there is a risk of injury to vehicle occupants. Conversely, this also means that it must not be deployed in a crash where deployment of the restraint system would not contribute toward a reduction in the risk of injury. It is essential that faulty deployment of the restraint system be prevented, because faulty deployment could result in frightening the driver and thus causing an accident or causing injury to a vehicle passenger due to the airbag itself. Faulty deployment must also be prevented because in some cases it is more expensive to replace an airbag after deployment than to repair the rest of the vehicle. In addition, the deployment algorithm should be as robust as possible. Robust is understood to refer to reliable deployment or nondeployment even in a crash not determined in advance by a crash test.
An object of the present invention is to provide a method and a device for forming a deployment criterion for a restraint system that is as robust as possible.
According to the present invention a deployment criterion for deciding whether or not the restraint system should be deployed is formed with the help of a fuzzy classifier which receives at least two input quantities. A first input quantity is an operating time which begins when the measured acceleration exceeds a predetermined threshold, and a second input quantity takes into account the change in vehicle speed determined from the measured acceleration. A very robust and reliable decision can be made regarding whether or not the restraint system should be deployed by using the fuzzy method with the abovementioned input quantities.
In one refinement of the present invention the second input quantity is formed from the change in vehicle speed based on the operating time to provide a robust and reliable method.
A further improvement in the method and/or the device with regard to its robustness and reliability is obtained when the fuzzy classifier receives a third input quantity corresponding to an average slope of the measured acceleration.